The present invention relates in general to sewing machines and in particular to a new and useful sewing machine with an electronic control unit for producing at least one stitch pattern which consists of a plurality of different single stitches with a RAM for entering into storage at least the single stitches of the stitch pattern, and with feed means whose feed adjustment is effected by a step motor controlled in accordance with stored data on the RAM during an inactive phase of the feed means.
In stitch patterns stored in electronic sewing machines, the stitch sequence is selected so that the variation of a step motor for the forward movement of the cloth feeder cannot exceed a certain amount.
For the free programming of stitch patterns by the operator it is necessary not only to limit the stitch length to be executed between the individual stitches to the maximum stitch length that the machine can execute, but also provision must be made that the time available to the step motor for execution of the displacement amount is not exceeded. Now in order to obtain a sufficiently fine gradation of the stitch length, the displacement of the step motor is divided into so large a number of steps that only the number of steps which corresponds to a certain limited stitch length variation, e.g. of 6 mm, can be executed with each stitch change. Owing to this, a displacement of the step motor from a relatively large forward stitch to a relatively large backward stitch cannot easily be executed within the time available at high sewing speeds.
When setting the step motor between two successive stitches, for example, from +4 mm to -3 mm, there results a total step displacement at the step motor with a number of steps which corresponds to a stitch length variation of 7 mm. For setting the step motor from stitch length +6 mm to the stitch length -6 mm there would result a number of steps which would correspond to a stitch length variation of 12 mm.
For such a step displacement to be possible, the sewing speed must be greatly reduced at least when this excessive number of steps occurs, otherwise it must be accepted that a stitch will be produced in the wrong place and the executed pattern will be distorted, because also the subsequent stitches will be shifted accordingly.
Another possibility would be that, if the operator enters in the program a stitch sequence with an excessive displacement of the step motor, the program control unit prodcues an indication of the non-feasibility of this pattern sequence. The operation, however, would then have to carry out a subsequent pattern change. Such a possibility does not exist if, after the pattern has been entered in the program, the pattern length can still be changed by the operator, because then the overstepping of the permissible number of steps of the step motor is dependent not only on the programmed data for the displacement of the work, but also on the data alteration entered later. In fact, depending on the selected pattern length, identical stitch sequences of the programmed pattern are then either possible or not possible.